Hans Brix

PHOSPHORUS REMOVAL BY SANDS FOR USE AS MEDIA IN SUBSURFACE FLOW CONSTRUCTED REED BEDS

Sorption of P to the bed sand medium is a major removal mechanism for P in subsurface flow constructed reed beds. Selecting a sand medium with a high P-sorption capacity is therefore important to obtain a sustained P-removal. The objective of this study was to evaluate the P-removal capacities of 13 Danish sands and to relate the removal to their physico-chemical characteristics. The P-removal properties were evaluated in short-term isotherm batch-experiments as well as in 12-week percolation experiments mimicking the P-loading conditions in constructed reed bed systems. The P-removal properties of sands of different geographical origin varied considerably and the suitability of the sands for use as media in constructed reed beds thus differs. The P-removal capacity of some sands would be used up after only a few months in full-scale systems, whereas that of others would persist for a much longer time. The most important characteristic of the sands determining their P-removal capacity was their Ca-content. A high Ca content favours precipitation with P as sparingly soluble calcium phosphates particularly at the slightly alkaline conditions typical of domestic sewage. In situations where the wastewater to be treated is more acid, the contents of Fe and Al may be more important as the precipitation reactions with these ions are favoured at lower pH levels. The maximum P-sorption capacities estimated using the Langmuir-isotherm plots did not correspond to or correlate with the actual amount of P removed in the percolation columns. Hence, the Langmuir-isotherm does not estimate the P-removal capacities of sands. It is suggested that a suitable quick method of screening a selection of potential media for P-removal potential is to perform simple removal isotherm studies using water with a similar chemical composition as the wastewater to be treated. This method will not provide a direct estimate of the P-removal capacity that can be obtained in full-scale systems, but it is a means of comparing the relative performance of potential media.

Phosphorus adsorption maximum of sands for use as media in subsurface flow constructed reed beds as measured by the Langmuir isotherm

The P-adsorption capacities of 13 Danish sands were studied by short-term isotherm batch experiments and related to the physico-chemical characteristics of the sands. The maximum P-adsorption capacities (Q) and P-binding energy constants (b) were calculated using the Langmuir-isotherm model. The Freundlich model was also used, but it was not useful for the description of adsorption phenomena per se since it fitted well P-removal data even if precipitation of P-salts occurred simultaneously. The Langmuir model described the data well (R(2)=0.90-0.99) when precipitation of phosphates did not occur and seems therefore to be useful for describing the adsorption processes per se. The relationships between maximum sorption capacities and physico-chemical characteristics of the sands were investigated using classical univariate and partial least squares regression analyses. Among the physico-chemical properties of the sands, Ca and Mg content, grain size, porosity, bulk density and hydraulic conductivity were significantly related (P<0.1) to the maximum adsorption capacity as estimated by the Langmuir model. Using the maximum P-adsorption capacities, it was estimated how long the P-removal can be sustained with the different sands in subsurface flow constructed reed beds. If the most efficient sand for P-adsorption was used, the adsorption capacity would be used up after about 1 year, while, for the less efficient sands, the P-retention would go on for about 2 months. This suggests that, in order to sustain a long-term P-removal in subsurface flow constructed reed beds, precipitation reactions of insoluble P-salts should be promoted. P-binding energy constants were not significantly related to the physico-chemical properties of the sands, except the Ca content, which showed, however, a low correlation coefficient.

Are Phragmites-dominated wetlands a net source or net sink of greenhouse gases?

Phragmites australis wetlands act as a sink for greenhouse gases by photosynthetic assimilation of carbon dioxide (CO2) from the atmosphere and sequestration of the organic matter produced in the wetland soil. The wetlands also act as a source for greenhouse gases by emission of sediment-produced methane (CH4) to the atmosphere. In P. australis wetlands, the dominant mechanism of CH4 release to the atmosphere is internal gas transport in the plants, primarily by pressurized convective gas flow. The time periods of carbon fixation and CH 4 release therefore vary seasonally and diurnally. The balance between net CO2-assimilation and CH4 emission determines if a wetland can be regarded as a net sink or a net source of greenhouse gases, and hence, the function of the wetland in relation to global climate change. On an annual basis up to 15% of the net carbon fixed by the wetlands may be released to the atmosphere as CH 4. Because of the different infrared absorption characteristics and atmospheric longevity of CH4 and CO2, the warming effect of CH4 in the atmosphere is about 21 times higher on a mass basis than CO2 over a 100-year timescale. Thus, the immediate carbon balance, coupled with the different physical characteristics of the two gases, would suggest that although some wetlands function as a net sink for CO2, the wetlands still increase the greenhouse effect because of their release of CH4. However, the short adjustment time for CH4 in the atmosphere means that, over a longer time scale, the radiative forcing of CH4 is less relative to CO2 and the wetlands effectively become a sink for greenhouse gases. Wetlands may therefore be regarded as a source for greenhouse gases and so increase radiative forcing if evaluated on a short time scale (decades), but as a sink for greenhouse gases and thus attenuating radiative forcing if evaluated over longer time scales (>100 years).

Development of constructed wetlands in performance intensifications for wastewater treatment: a nitrogen and organic matter targeted review.

The knowledge on the performance enhancement of nitrogen and organic matter in the expanded constructed wetlands (CWs) with various new designs, configurations, and technology combinations are still not sufficiently summarized. A comprehensive review is accordingly necessary for better understanding of this state-of-the-art-technology for optimum design and new ideas. Considering that the prevailing redox conditions in CWs have a strong effect on removal mechanisms and highly depend on wetland designs and operations, this paper reviews different operation strategies (recirculation, aeration, tidal operation, flow direction reciprocation, and earthworm integration), innovative designs, and configurations (circular-flow corridor wetlands, towery hybrid CWs, baffled subsurface CWs) for the intensifications of the performance. Some new combinations of CWs with technologies in other field for wastewater treatment, such as microbial fuel cell, are also discussed. To improve biofilm development, the selection and utilization of some specific substrates are summarized. Finally, we review the advances in electron donor supply to enhance low C/N wastewater treatment and in thermal insulation against low temperature to maintain CWs running in the cold areas. This paper aims to provide and inspire some new ideas in the development of intensified CWs mainly for the removal of nitrogen and organic matter. The stability and sustainability of these technologies should be further qualified.

GAS FLUXES ACHIEVED BY IN SITU CONVECTIVE FLOW IN PHRAGMITES AUSTRALIS

The Common Reed (Phragmites australis Cav. Trin. ex Steud.) possesses an outstanding capacity to vent its underground tissues by pressurized through-flow. Phragmites-dominated wetlands therefore potentially provide a significant source of trace gas emissions to the atmosphere. In this paper we present results of in situ studies on gas exchange through Phragmites, and evaluate various methodologies used for measuring gas transport and the fluxes they record. Gas exchange rates were related to atmospheric humidity, temperature and light. Green shoots were influx culms and dead culms and broken or damaged green shoots were efflux culms. Gas exchange through the plants fluctuated diurnally, with highest rates in the early afternoon (up to 11 l m−2 h−1) and lowest rates during the night. The net flux of O2 to the below-ground tissues and sediment was up to 5.7 1 m−2 day−1, and the net emissions of CO2 and CH4 up to 5.1 and 0.27 l m−2 day−1 respectively. Net gas exchange rates varied with season and sediment characteristics, being highest during hot and dry summer days, and on organic sediments with a high oxygen demand and high rates of microbial decomposition. Hence, the convective throughflow mechanism in Phragmites not only accelerates gas exchange between the sediment and the atmosphere, but the oxygen delivered through the plant may also affect the microbial processes in the sediment. Therefore, the role of the plants for rhizosphere oxidation and conveyers of gases should be further assessed in future studies. A comparison of current methods for measuring flow suggested that they need refining if they are to quantify gas exchange through Phragmites wetlands on a large scale or over longer time periods.

Preliminary screening of small-scale domestic wastewater treatment systems for removal of pharmaceutical and personal care products

Occurrence and removal efficiencies of 13 pharmaceuticals and personal care products (PPCPs) as well as BOD(5), TSS and NH(4)(+) were evaluated for the first time in thirteen onsite household secondary wastewater treatment systems, including two compact biofilters followed by Filtralite-P filter units, two biological sand filters, five horizontal subsurface flow and four vertical flow constructed wetlands. As expected, all systems removed TSS and BOD(5) efficiently (>95% removal). The PPCP removal efficiencies exceeded 80% with the exception of carbamazepine, diclofenac and ketoprofen because of their more recalcitrant characteristics. Despite no statistical differences in the PPCP removal were observed between the different systems evaluated, the vegetated vertical flow constructed wetlands which had unsaturated flow and hence better oxygenation, appeared consistently to perform better in terms of PPCP removal efficiency. The combined effects of vegetation and unsaturated water flow provide a higher tolerance to variations in loading rate and a consistent removal rate.

SOIL OXYGENATION IN CONSTRUCTED REED BEDS: THE ROLE OF MACROPHYTE AND SOIL-ATMOSPHERE INTERFACE OXYGEN TRANSPORT

The in- and efflux of metabolic gases through the soil-atmosphere interface and through the hollow culms of reed (Phragmites australis) in a soil-based constructed reed bed with lateral sub-surface water flow was quantified. The total flux of gaseous oxygen into the bed substrate was 5.86 g m-2day-1 of which 2.08 g m-2 day-1 was through the hollow culms of standing-dead culms of P. australis. The respiratory oxygen consumption of roots and rhizomes almost perfectly balanced the oxygen influx through the culms leaving only 0.02 g O2 m-2day-1 to be released to the surrounding soil. The macrophyte-induced rhizosphere oxygenation was therefore of no quantitative importance for aerobic BOD degradation and microbial nitrification. The major drawbacks of the design are the lack of ability of the reeds to develop a sufficiently high hydraulic conductivity of the soil and to transfer oxygen into the substrate. Subapical regions of white young roots of P. australis, Glyceria maxima, Typha latifolia and Iris pseudacorus released oxygen, whereas no release was detected from old roots and rhizomes Studies on the potential diffusive oxygen transfer capacity of reeds showed that at 15° C the respiratory oxygen demand of the root-system would balance the diffusive transport capacity for root lengths of approx. 60 cm. The oxidation in constructed reed beds can be significantly improved by changes in the design and loading regime. In a vertical flow system consisting of several beds laid out in parallel with intermittent water loading, the oxygen transfer from the atmosphere to the bed substrate would be 30 to 150 g m-2day-1 depending on substrate texture.

Internal gas transport in Typha latifolia L. and Typha angustifolia L. 1. Humidity-induced pressurization and convective throughflow

The internal gas transport in the shoots of the cattails, Typha latifolia L. and Typha angustifolia L., occurs principally via pressurized convective throughflow of gases. Static pressure differentials of up to 350 Pa relative to ambient for T. latifolia and 570 Pa for T. angustifolia were found to be generated mainly by humidity-induced diffusion at ambient temperatures of 15–25°C. Thermal transpiration did not contribute significantly to the internal pressurization. Convective gas flow rates of up to 8 cm3 min−1 for T. latifolia and 3.5 cm3 min−1 for T. angustifolia were recorded from cut rhizomes. Internal pressurization and convective throughflow rates were highest at high ambient temperature and low ambient relative humidity. Light did not affect pressurization in T. latifolia, whereas pressurization and convective gas flows were lower in the light than in the dark in T. angustifolia probably as a consequence of stomatal movements. A layer of closely packed mesophyll cells located just below the palisade parenchyma of the leaves is probably the porous partition responsible for the pressurization, but stomata with tortuous pathways may also be involved. Under identical environmental conditions the ventilation capacity of T. angustifolia is about twice as high as that of T. latifolia indicating that root aeration of the former may be more efficient.

Growth and root oxygen release by Typha latifolia and its effects on sediment methanogenesis

Growth of Typha latifolia L. and its effects on sediment methanogenesis were examined in a natural organic sediment and a sediment enriched with acetate to a concentration of 25 mM in the interstitial water. The lower redox potential and higher oxygen demand of the acetate-enriched sediment did not significantly impede growth of T. latifolia despite some differences in growth pattern and root morphology. Plants grown in acetate-enriched sediment were ca. 15% shorter than plants grown in natural sediment, but the former produced more secondary shoots at earlier stages, which resulted in similar total biomasses after 7 weeks of growth in the two sediment types. Plants grown in acetate-enriched sediment had thicker and much shorter roots than plants grown in natural sediment. This difference did not significantly affect the release of oxygen from the roots when measured under laboratory conditions, which was 0.12‐0.20 mmol O2 g ˇ1 DW h ˇ1 . Enrichment with acetate resulted in much higher sediment methanogenesis rates (643 vs. 90 nmol CH4 g ˇ1 sediment DW h ˇ1 ). Growth of T. latifolia significantly reduced methanogenesis in both types of sediment, but the effect was twice as marked in the natural sediment (34%) as in the acetateenriched sediment (18%), although in absolute terms the reduction was higher in the enriched sediment. The data suggest that this effect of plant growth was via root oxygen release and its effect on redox conditions. In the natural sediment, oxygen release resulted in a significantly higher redox potential and lower sediment oxygen demand, whereas there were no significant changes in the acetate-enriched sediment. The very high oxygen demand of this sediment probably masked the effect of root oxygen release so that a significant reduction in methanogenesis occurred without any significant increase in the redox potential. This demonstrates how root oxygen release from plants like T. latifolia can significantly alter rates of biogeochemical processes such as methanogenesis,

Gas exchange through the soil-atmosphere interphase and through dead culms of Phragmites australis in a constructed reed bed receiving domestic sewage.

The decomposition processes of organic matter during winter in a 3-year old soil-based constructed reed bed with lateral sub-surface water flow were evaluated by quantifying the in- and efflux of metabolic gases and by considering input-output budgets and relating these to storages within the bed. The total flux of gaseous oxygen into the bed was 5.86 g m−2 day−1 of which 2.08 gm−2day−1 was through the cavities of dead, still standing culms of Phragmites australis. The respiratory oxygen consumption of the roots and rhizomes almost perfectly balanced the oxygen influx through the culms leaving only 0.02 g O2 m−2 day−1 to be released to the surrounding soil. The macrophyte-induced rhizosphere oxygenation was therefore of no quantitative importance for aerobic BOD degradation and microbial nitrification. Organic matter was degraded aerobically by means of oxygen delivered directly from the atmosphere and anaerobically by methanogenic bacteria in the upper layers of the soil. Some nitrogen and phosphorus was retained with sludge deposition on the surface of the bed and in the inlet trench. The constructed reed bed under study did not function according to the theory of the root-zone process. The majority of the wastewater moved along preferred pathways on the surface of the bed because of low soil hydraulic conductivity. Changes in the physical design and the operational practice of constructed reed beds needed to improve and optimize performance efficiency for nutrients are discussed.